1. Field of the Invention
The present invention relates to a digital image, and more particularly to a device that inserts identification data, which has special information, into a digital image and a device that detects the identification data.
2. Description of the Related Art
Recently, more and more data recorded on media is digitized. On the other hand, an illegal copy of data, brought by data digitization, has become a serious social problem. Electronic watermark (hereinafter called a watermark) insertion and detection technology, designed for preventing illegal copies, is now being studied for practical use. Watermark technology, a technology for embedding a sort of invisible ID information as a noise, is characterized in that embedded information that constantly coexists with contents cannot be erased or modified easily. Taking advantage of these characteristics, watermark insertion/detection technology prevents contents, such as video data, from being illegally copied.
As an example of electronic watermark technology, a method for embedding a watermark is proposed in which, after an image is frequency-converted, the watermark is embedded into an area where the frequency of video signal components is high. Because a watermark is embedded into a high-frequency component area in this method, the watermark will not be removed even if image processing, such as compression/decompression and filtering, is performed. The watermark embedded in this way is removed only when the original image is destroyed. In addition, arranging watermarks based on random numbers generated according to normal distribution avoids interference among watermarks, preventing image quality from being degraded.
This method embeds a watermark in the following steps. First, the original image is converted to frequency components using, for example, DCT (discrete cosine transform), and n data pieces, f(1), f(2), . . . , f(n), each high in the frequency region, are selected. Then, watermarks, w(1), w(2), . . . ,w(n) are selected from those arranged according to normal distribution (average is 0, covariance is 1) and, for each i, the following calculation is executed.F(i)=f(i)+α×|f(i)|×w(i)where, α is a scaling element.
Then, performing inverse DCT for F(i) gives an image in which a watermark is embedded.
This method detects a watermark in the following steps. This method requires that the original image f(i) and a watermark candidate w(i) (where, i=1, 2, . . . , n) be known.
First, an image with a watermark embedded is converted to frequency components using DCT. Let F(1), F(2), . . . , F(n) be the values of elements corresponding to f(1), f(2), . . . , f(n) each of which has a watermark embedded in the frequency region. A watermark W(i) is calculated and extracted using f(i) and F(i) as follows:W(i)=(F(i)−f(i))/f(i)
Next, the statistical similarity between w(i) and W(i) is calculated using the inner product of the vector as follows:C=W w(WD×wD)where,W=(W(1), W(2), . . . , W(n)),w=(w(1), w(2), . . . , w(n)),
WD is the absolute value of vector W, wD is the absolute value of vector w, and is the inner product of the vector. When the statistical similarity C is a value equal to or larger than a specific value, it is judged that the watermark is embedded.
If a watermark is embedded in this method, the copyright holder of the original image may find the source of digital image data that is illegally copied. This method, which requires an original image, allows the copyright holder to detect a watermark only when he or she has the original image of image data which is thought to be copied illegally. However, on a terminal reproducer where the original image is not available, this method cannot be used to detect a watermark.
To solve this problem, a method improved for use on a terminal, especially for use in an MPEG system, is proposed. This method divides the original image into 8×8 pixel blocks and embeds and extracts a watermark into and from those blocks, one block at a time.
This method embeds a watermark in the following steps. First, let f(1), f(2), . . . , f(n) be the frequency components in the frequency region, arranged in AC frequency ascending order, for which discrete cosine transfer has been performed during MPEG compression. Then, watermarks w(1), w(2), . . . , w(n) are selected from those arranged according to normal distribution (average is 0, covariance is 1) and, for each i, the following calculation is executed,F(i)=f(i)+α×avg(f(i))×w(i)where, α is a scaling element, and avg(f(i)) is a partial average of the absolute values in three points near f(i).
Then, processing that follows MPEG processing is performed using F(i) instead of f(i).
This method detects a watermark in the following steps. This method does not require the original image; only the watermark candidates w(i) (where, i=1, 2, . . . , n) need be known.
First, let F(1), F(2), . . . , F(n) be the frequency components in the frequency region, arranged in frequency ascending order, for which de-quantization has been performed during MPEG decompression. With the absolute value of the average of three points near F(i), that is, F(i−1), F(i), and F(i+1), as the partial average avg(F(i)), watermark W(i) is calculated from W(i)=F(i)/avg(F(i)) and, for each i, the total WF(i) of W(i) for one image is calculated.
Then, the statistical similarity of w(i) and WF(i) is calculated from C=WF w/(WFD×wD) using the inner product of the vector. When the statistical similarity C is a value equal to or larger than a specific value, it is judged that the watermark is embedded.
FIG. 8 shows the configuration of a device that inserts an electronic watermark into MPEG-compressed image data. In the figure, numeral 802 indicates a DCT transformer that performs DCT (discrete cosine) transformation for an original image 801 and outputs DCT-transformed data, numeral 803 indicates a watermark inserter that puts watermark weighs on the DCT coefficients as described above, numeral 804 indicates a quantizer that quantizes the DCT coefficients into which a watermark is inserted, numeral 805 indicates a de-quantizer that de-quantizes quantized data, numeral 806 indicates an IDCT transformer that performs IDCT(inverse discrete cosine transform) for de-quantized data, numeral 807 indicates an image into which a watermark is inserted, numeral 808 indicates a Huffman encoder that performs Huffman coding to compress quantized data, and numeral 809 indicates data compressed through Huffman encoding. The device with this configuration inserts a watermark into the original image 801 and then provides general users with the compressed data 809 into which a watermark is inserted.
FIG. 9 shows the configuration of a device that decodes the contents into which a watermark is inserted. In the figure, numeral 902 indicates a decoder that decodes compressed data 901 into which a watermark is inserted, numeral 903 indicates an IDCT transformer that performs IDCT for decoded data, and numeral 904 indicates a watermark detector that detects a watermark in data for which IDCT has been performed as described above. The device with this configuration detects a watermark inserted in the contents.
On the other hand, the configuration of a watermark is shown in FIG. 10. The high-order four bits of an eight-bit watermark contains information defined by the electronic watermark promotion organization. More specifically, the high-order two bits are defined as the CCI (copy protection) bits and bits 3–4 are reserved. The low-order four bits are undefined.
The use of only the high-order four bits are defined with the low-order four bits undefined as described above. How to use the remaining low-order four bits, reserved for future use, is a problem.